Generate Parentheses

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

"((()))", "(()())", "(())()", "()(())", "()()()"

class Solution {
public:
void helper(vector<string>& svec, string s, int left, int right)
{
if(left > right)
{
return;
}
if(left == 0 && right == 0)
{
svec.push_back(s);
return;
}
if (left > 0)
{

helper(svec, s + '(', left - 1, right);
}

if(right > 0)
{
helper(svec, s + ')', left, right - 1);
}
}
vector<string> generateParenthesis(int n)
{
vector<string> svec;
string s="";
helper(svec, s, n, n);
return svec;
}
};